possible infinite loop with `group-by` + `ap`

[leonoel]

@mjmeintjes via slack :

(m/ap
     (let [[g fl] (->> (m/seed [1 2 1 1 3 1])
                       (m/group-by (comp keyword str))
                       m/?=)
           i (m/?> fl)]
       (when (= g :2)
         (throw (ex-info "GROUP 2" {})))
       (println "GROUP" g "  VAL " i)
       [g i]))

When the first exception is thrown, the process is cancelled, then

1. the next group is transferred
2. group consumer is run
3. group consumer is cancelled immediately, because parent process is in cancelled state
4. group-by creates a new group for the unconsumed value, goto 1

[leonoel]

A viable solution would be to change group-by such that after the process is cancelled, cancelling any of its consumers has no effect.

[leonoel]

Fixed in b.26